Metal foam structures are porous, three-dimensional structures with a variety of uses, including medical implants. Metal foam structures are suitable for medical implants, particularly orthopedic implants, because they have the requisite strength for weight bearing purposes as well as the porosity to encourage bone/tissue in-growth. For example, many orthopedic implants include porous sections that provide a scaffold structure to encourage bone in-growth during healing and a weight bearing section intended to render the patient ambulatory more quickly.
Metal foam structures can be fabricated by a variety of methods. For example, one such method is mixing a powdered metal with a pore-forming agent (PFA) and then pressing the mixture into the desired shape. The PFA is removed using heat in a “burn out” process. The remaining metal skeleton may then be sintered to form a porous metal foam structure.
Another similar conventional method include applying a binder to polyurethane foam, applying metal powder to the binder, burning out the polyurethane foam and sintering the metal powder together to form a “green” part. Binder and metal powder are re-applied to the green part and the green part is re-sintered until the green part has the desired strut thickness and porosity. The green part is then machined to the final shape and re-sintered.
While metal foams formed by such conventional methods provide good porosity, they may not provide sufficient strength to serve as weight bearing structures in many medical implants. Further, the processes used to form metal foams may lead to the formation of undesirable metal compounds in the metal foams by the reaction between the metal and the PFA. Conventional metal foam fabrication processes also consume substantial amounts of energy and may produce noxious fumes.
Rapid manufacturing technologies (RMT) such as direct metal fabrication (DMF) and solid free-form fabrication (SFF) have recently been used to produce metal foam used in medical implants or portions of medical implants. In general, RMT methods allow for structures to be built from 3-D CAD models. For example, DMF techniques produce three-dimensional structures one layer at a time from a powder which is solidified by irradiating a layer of the powder with an energy source such as a laser or an electron beam. The powder is fused, melted or sintered, by the application of the energy source, which is directed in raster-scan fashion to selected portions of the powder layer. After fusing a pattern in one power layer, an additional layer of powder is dispensed, and the process is repeated with fusion taking place between the layers, until the desired structure is complete.
Examples of metal powders reportedly used in such direct fabrication techniques include two-phase metal powders of the copper-tin, copper-solder and bronze-nickel systems. The metal structures formed by DMF may be relatively dense, for example, having densities of 70% to 80% of a corresponding molded metal structure, or conversely, may be relatively porous, with porosities approaching 80% or more.
While DMF can be used to provide dense structures strong enough to serve as weight bearing structures in medical implants, such structures do not have enough porosity to promote tissue and bone in-growth. Conversely, DMF can be used to provide porous structures having enough porosity to promote tissue and bone in-growth, but such porous structures lack the strength needed to serve as weight bearing structures. Other laser RMT techniques are similarly deficient for orthopedic implants requiring strength, porosity and connectivity.
As a result of the deficiencies of metal foam implants and implants fabricated using conventional DMF methods, some medical implants require multiple structures, each designed for one or more different purposes. For example, because some medical implants require both a porous structure to promote bone and tissue in-growth and a weight bearing structure, a porous plug may be placed in a recess of a solid structure and the two structures may then be joined by sintering. Obviously, using a single structure would be preferable to using two distinct structures and sintering them together.
In light of the above, there is still a need for porous implant structures that provide both the required strength and desired porosity, particularly for various orthopedic applications. This disclosure provides improved porous structures that have both the strength suitable for weight bearing structures and the porosity suitable for tissue in-growth structures and a method for fabricating such improved porous structures.